1. Field of the Invention
The invention relates to counter rotating aircraft gas turbine engines with counter rotating fans driven by counter rotating low pressure turbine rotors and, particularly, for such engines having a single direction of rotation booster downstream of the counter rotating fans and incorporating vanes to effect unequal power splits and variable torque between the counter rotating low pressure turbine rotors.
2. Description of Related Art
A gas turbine engine of the turbofan type generally includes a forward fan and booster, a middle core engine, and an aft low pressure power turbine. The core engine includes a high pressure compressor, a combustor and a high pressure turbine in a serial flow relationship. The high pressure compressor and high pressure turbine of the core engine are interconnected by a high pressure shaft. The high pressure compressor, turbine, and shaft essentially form the high pressure rotor. The high pressure compressor is rotatably driven to compress air entering the core engine to a relatively high pressure. This high pressure air is then mixed with fuel in the combustor and ignited to form a high energy gas stream. The gas stream flows aft and passes through the high pressure turbine, rotatably driving it and the high pressure shaft which, in turn, rotatably drives the compressor.
The gas stream leaving the high pressure turbine is expanded through a second or low pressure turbine. The low pressure turbine rotatably drives the fan and booster via a low pressure shaft, all of which form the low pressure rotor. The low pressure shaft extends through the high pressure rotor. Some low pressure turbines have been designed with counter rotating turbines that power counter rotating fans and counter rotating boosters or low pressure compressors. U.S. Pat. Nos. 4,860,537, 5,307,622, and 4,790,133 disclose counter rotating turbines with counter rotating rotors that power counter rotating fans and boosters. Most of the thrust produced is generated by the fan. Blade rows or stages of one of the counter rotating turbines, turbine rotor are interdigitated with blade rows or stages of another of the counter rotating turbine rotors. No vanes are disposed between the interdigitated rows of blades. A radially outer drum supports blade rows of one of the counter rotating turbines. These blade rows depend radially inwardly from the drum.
Advanced commercial gas turbine engines having counter rotating forward and aft fans and counter rotating boosters are being designed. It is desirable to design a counter rotating engine with a peak performance. It has been found that a peak performance can be attained when the forward fan operates at a higher fan pressure ratio and higher rotational speed than the aft fan. This can result in a substantial mismatch in horsepower and rotational speed between the counter rotating rotors. The counter rotating low pressure turbine is required to supply the necessary power to each of the forward and aft fans at the rotational speed of each fan. A conventional counter rotating turbine will operate at peak efficiency when the power split between both shafts is equal and when the rotational speeds are equal and opposite. In such a case, speed and horsepower ratios of the two rotors and turbines are substantially 1. It is highly desirable to have a gas turbine engine with counter rotating low pressure turbines that have different speed and horsepower ratios such as speed ratio of about 1.20 and a horsepower ratio below 1.1 to attain good fan efficiency.
A gas turbine engine turbine assembly includes a high pressure spool having a high pressure turbine drivingly connected to a high pressure compressor by a high pressure shaft which is rotatable about an engine centerline. A low pressure turbine includes a low pressure turbine flowpath and is located aft of the high pressure spool. The low pressure turbine has counter rotatable low pressure inner and outer shaft turbines drivingly connected to coaxial low pressure inner and outer shafts respectively which are at least in part rotatably disposed coaxial with and radially inwardly of the high pressure spool. The low pressure inner shaft turbine including first low pressure turbine blade rows is disposed across the low pressure turbine flowpath and is drivingly connected to a forward fan blade row by the low pressure inner shaft. The low pressure outer shaft turbine including second low pressure turbine blade rows is disposed across the low pressure turbine flowpath and is drivingly connected to an aft fan blade row by the low pressure outer shaft. At least one row of low pressure variable vanes is disposed between the counter rotating low pressure turbines. A single direction of rotation booster is drivenly connected to the low pressure outer shaft and axially located aft and downstream of the aft fan blade row. The booster has at least a rotatable first row of booster blades. The single direction of rotation booster as opposed to counter rotational boosters allow the counter rotating low pressure turbines to operate at different speed and horsepower ratios to attain good fan efficiency.
The counter rotatable low pressure inner and outer shaft turbines may be interdigitated such that the first low pressure turbine blade rows interdigitated with the low pressure second turbine blade rows. Alternatively, the low pressure inner and outer shaft turbines may be tandem non-interdigitated aft and forward low pressure turbines, respectively, in which the aft low pressure turbine is located aft and downstream of the forward low pressure turbine.
One exemplary embodiment of the assembly includes a core engine inlet leading to the high pressure compressor and the booster is operably disposed entirely within the core engine inlet to direct substantially all booster air from the booster into the high pressure compressor. Forward and aft rows of booster vanes may be axially disposed forwardly and aftwardly respectively of the first row of booster blades. The first and a second (or more) rows of booster blades of the booster may be axially disposed between forward and aft booster vanes. At least one middle row of booster vanes is axially disposed between each pair of the rows of booster blades.
Another exemplary embodiment of the assembly has the core engine inlet located downstream and axially aft of the booster. The core engine inlet has an inlet duct splitter axially and radially disposed adjacent to and downstream of the booster for splitting booster air from the booster into booster air first and second portions. The inlet duct splitter is positioned for directing the booster air first portion into the core engine inlet and the booster air second portion around the core engine inlet. The booster includes at least one row of booster blades surrounded by a splitter shroud having a leading edge splitter which is operably disposed adjacent to and downstream of the aft fan blade row for splitting fan flow air exiting the aft fan blade row into a fan flow air first portion into the booster and a fan flow air second portion around the booster. Forward and aft rows of booster vanes may be disposed forwardly and aftwardly of the booster blades, respectively. The aft row of booster vanes may have radially inner vane portions disposed within the core engine inlet and radially outer vane portions disposed between the splitter shroud and a core engine inlet shroud which includes the inlet duct splitter.
The single direction of rotation booster drivenly connected to the low pressure outer shaft and axially located aft and downstream of the counter rotating fans allows a gas turbine engine with counter rotating low pressure turbines having at least one row of low pressure variable vanes disposed between the counter rotating low pressure turbines to operate with different speed and horsepower ratios in order to attain good fan efficiency. One example of such ratios are a speed ratio of about 1.20 and a horsepower ratio below 1.1. The single direction of rotation booster also eliminates cantilevered interdigitized booster blades and thus allows a more efficient engine and a more efficient, mechanically less complicated, and robust design of the fan and booster system.